Conventional cellular radiotelephone systems attempt to conserve power consumption of a subscriber unit by decreasing the transmit power of the subscriber unit (mobile or portable) during discontinuous voice transmissions from the subscriber unit to a cell site. However, a predetermined set of supervisory audio tones (SAT) is also typically modulated with the voice signal (constituting a composite signal). The SATs are used to maintain continuity with cell sites during voice transmission, and to differentiate the intended subscriber unit from surrounding subscriber units. Discontinuous transmission generally causes the cell site to lose communication with the mobile once the voice (audio) ceases due to the signal strength falling below a detectable level. The cell site therefore has virtually no information as to whether the mobile (source) is within communication distance or whether it intends to re-establish communication.
If the cell site does not receive the SAT for a predetermined period of time, the cell site typically attempts to re-establish link continuity via an audit message. In addition, a destination unit (landline or other mobile), after no longer receiving audio from the mobile, may continually receive noise from the voice channel unless the cell site mutes the audio on that channel after detecting that the mobile has dropped its power level. Although the problems of inefficient power consumption and maintaining link continuity during discontinuous transmissions are evident in cellular communication systems, they also adversely affect numerous other communication systems which desire efficient power control or communication link continuity.
Accordingly, there exists a need to maintain communication between a subscriber unit (or first communication unit) and a cell site (or second communication unit) during discontinuous transmission while also limiting power consumption by the subscriber unit. In addition, timely detection of discontinuous transmission and transmission must be provided to allow prompt muting and unmuting of audio signals thereby substantially reducing undesirable audio noise interference and preventing loss of valid audio signals.